Many solutions have been proposed for the problem of endoprosthetics, which permit unchangeable maintenance of a stable bone implantate joint, which will last for a very long time, ideally from implantation to the death of the patient or wearer of the prosthesis. There are, primarily, two factors which interfere with such lifetime implantation. For one, the interface between the bone-implantate is subject to changeable forces, which change both with respect to value as well as direction. Particularly shearing forces are involved. For another, biological reactions of tissues and degrading bones are a factor, especially reactions to foreign-body materials, and reactions to abraded particles from the prosthesis itself.
The long lifetime, and particularly implanted lifetime, of a joint can be increased by reducing changeable forces engaging at the interface between the various prosthetic parts, and on the sliding surfaces thereof. The wear on the sliding surfaces should be minimized. Various solutions have been proposed, but not all of them can be applied at the same time. It is not sufficient to consider all the technical aspects of the joint; anatomical as well as physiological changeable conditions must be considered. The complex kinematic which occurs in joints, and particularly in complex joints such as the knee joint, can make it difficult to compromise between conflicting solutions applicable to specific parts, to achieve the overall goal of lifetime reliability.
It is well known that the wear and tear on slide bearings can be reduced by decreasing the per-square or area pressure of the respectively rubbing sliding parts. The surface or area pressure, and the resulting wear and tear on the bearing, is small when the contact surface of the two sliding elements is large. This contact bearing surface can be increased by making the sliding surfaces as large as possible and effectively congruent. Typical examples for such bearings are shown in FIG. 1, in which a straight slide bearing is schematically illustrated, and in FIG. 2, which, schematically, illustrates a hinge or ball joint. The slide bearing of FIG. 1, in principle, can be considered as a hinge or ball joint in which the engaging surfaces have infinite radii. Yet, in spite of the common characteristic of minimal wear and tear, there are basic differences: The slide joint, FIG. 1, is free for translatory movement, but does not have an axis of rotation. The hinge or ball joint, on the other hand, has an axis of rotation, but is restrained from translatory movement. This results in different relationships with respect to externally acting forces.
Reference is again made to FIG. 1:
A force, such as force F1 coming from above at an inclination, results in lateral shifting of the sliding head due to horizontal component of the force. This horizontal component of the force does not have any effect on the lower part of the sliding joint.
A force similar to force F1, when applied to a hinge or ball joint, see FIG. 2, passes through the joint without causing any rotation thereof. The horizontal component of this force, however, results in an undesired shear force, which continuously changes its direction if the upper portion of the joint oscillates back and forth like a pendulum. Most simply, the hinge joint has the advantage of rotary movement, which, however, is obtained by accepting the disadvantage of translatory immobility, and shearing forces at the interface.
Human body joints rarely are pure hinge joints or pure slide joints. Usually, and especially the knee joint provides a combination of both. A rotary movement and translatory movement can be superimposed upon each other.
In order permit a combination of such movements, it is necessary to open the hinge joint, that is, it is necessary to reduce the congruence of the slide surfaces with respect to each other. Referring to FIG. 3, the contact surfaces are decreased which, however, substantially increases the area or surface pressure, and hence wear and tear on the joint. This wear and tear is further increased by repetitive translatory movement of the head of the joint on the slide surface. Due to the almost point or at best somewhat line contact, and hence a similar high surface pressure, a kneading process on the respective joint parts results which, in turn, causes material fatigue of the lower joint portion or component. The non-congruent position of the elements does not necessarily prevent the occurrence of shear forces at the interface. Raised slide surfaces at the interface result in forces which are similar to those which arise in a pure hinge or ball joint. The result is increased wear and tear as well as shear forces at the interface, which has undesirable effects in all prostheses of this type.
It has been proposed to use a combination of slide and ball joints, in which the sliding and ball joint components are located on two different planes, by using an intermediate element, see FIG. 4. The rotary movement of the ball joint and a translatory movement of the slide joint are vertically staggered, so that the translatory movement is available as a lower motion of the ball joint. The congruence of joint surfaces is fully retained; wear and tear is minimized. The forces supplied by the ball joint are not transferred to the interface but, rather, translated into translatory movement by the intermediate element. Prostheses are of this type are known, and have been referred to as meniscal knees.
U.S. Pat. No. 4,309,778, Buechel and Pappas, describes two different knee joint prostheses, which have also been referred to as the "Oxford Knee" and the "New Jersey Knee". The "Oxford Knee" has two femoral portions, two intermediate portions and two tibial portions. The femoral portions each have a spherical segment which has a retention section, for retention on the femur. The tibial parts also have retention on the tibia. The tibial parts not only have retention sections for attachment to the tibia, but also a flat plateau on which the intermediate part can slide.
Upon flexion of 90.degree. and more, the intermediate part can slide over the flat or table-like surface and, possibly, can be entirely dislocated. A similar danger may occur if the tension of the remaining ligaments decreases after the operation and the femoral part lifts off the intermediate part. A new operation of the knee joint will then become necessary.
The "New Jersey Knee", described for example in FIG. 15 of the referenced U.S. Pat. No. 4,309,778, has a special arrangement to prevent loss of contact over the table or slide surface. Two dovetail-like, bowed grooves are provided on the tibia part; an engagement element is provided for the intermediate part element, fitting in the grooves through which the intermediate part element are constrained to be guided. The curves are directed towards the center, so that the intermediate part, upon bending of the knee, cannot slide backwardly in uncontrolled manner, and pushed over the surface or table, but, rather, engages the still remaining central projection from the bone. The dovetaile shape prevents dislocation or luxation of the joint, if the ligaments should lose tension or will have decreased tension or strength.
The intermediate portions are constrained to be guided on a predetermined path which permits congruence of the joint surfaces between the femoral portion and the intermediate portion only in a single position of the joint. For all other positions of the joint than the single one, an incongruence of joint surfaces results, which increases wear and tear of the engaging surfaces. Non-congruent conditions arise because the femoral parts, which are securely anchored in the femur, are always at the same distance from each other and have a common axis of rotation. The intermediate parts, however, approach each other laterally upon sliding forwardly and backwardly on the curved path or, respectively, separate from each other. Further, their axes of the hinge or ball joints continuously change their position with respect to each other. Similar situations obtain, in reverse sense, however, upon rotary movement about an axis which is perpendicular to the slide surface. The continuously changing degree of non-congruence of the surfaces of this prosthesis, and particularly of the hinge or ball joint surfaces, causes increased wear and tear and decreases the effect of a meniscal layer in a knee joint.
U.S. Pat. No. 4,353,136, Polyzoides et al, describes an endoprosthetic knee joint having a femoral, a tibial part and an intermediate part. The femoral part has an attachment portion for attachment to the femur and two condyles. The tibial part has an attachment portion and projecting attachment ribs for attachment to the tibia. The side opposite the attachment surface has a flat bearing surface, with a groove to receive a rib of an intermediate part. The rib of the intermediate part fits into the curved groove of the tibial part; two concave slide bearings provide counter surfaces for the condyles of the femoral part. This prosthesis has the advantage of congruence of the slide surfaces of a hinge or ball joint, and thus of low wear and tear.
The curved groove and rib coupling does not, however, permit translatory movement of the femoral part relative to the tibial part. It only permits rotation about the axis of the curve of the groove. This, then, is a classic hinge joint with additional freedom of rotation about an axis perpendicular to the hinge axis. Thus, forces which come at an inclination from above, see FIGS. 2-4, which occur, for example, due to muscle pull or loading upon placing the foot of the wearer on the ground, will pass through the joint to the interface and there result in the undesired shearing forces. The principle of a meniscal knee is compromised in that the congruence of the joint surfaces, which reduces wear and tear, is obtained only by loss of the translatory capability, which protects the connection between the prosthesis and the joint. The natural knee kinematics, thus, are not entirely obtained with this joint. Besides that, the use of elements which prevent dissociation or luxation of the joint, for example at dovetail interconnection, is nearly impossible, especially to permit insertion of the intermediate portion at a later time, after the tibial portion or part has already been implanted. This has the disadvantages which have been discussed above in connection with the "Oxford Knee".
British Patent 1,567,007, Minns et al, describes a knee joint having a femoral part, a tibial part and an intermediate part. The femoral part, as is customary, has an attachment element and a condyle. The tibial part also has an attachment element for connection to the tibia and a straight, dovetaile-shaped groove to receive a matching rib of the intermediate part. The intermediate part has a straight rib, fitting into the groove. The intermediate part, further, has a shallow glide bearing for articulation with the condyle of the femoral part. This structure also has the advantage of wear reducing congruence of the slide surfaces. The dovetail connection between rib and groove prevents luxation or dissocation of the intermediate part, and hence undesired separation from the tibial part.
This structure permits translatory movement in the longitudinal direction of the groove. It does not permit rotational movement about an axis perpendicular to the plane of the tibial part, that is, essentially perpendicularly to the axis of the tibia. Such rotary movements, however, are superimposed practically with all movements of the knee joint, which results in continuous non-congruent engagement of the slide surfaces of the condyle from the femur and the intermediate part. The natural kinematics or relative movements of the parts of the knee joint are thus not adequately reproduced.